Measurement of the individual factors that drive energy balance and tissue macronutrient fuel choice is fundamental to understanding the mechanisms influencing food intake behavior, energy expenditure, and ultimately, weight gain and body composition. Such knowledge is directly relevant to pressing public health issues such as obesity, type 2 diabetes mellitus, metabolic syndrome, and others for which risk is associated with metabolic dysfunction and long-term positive energy balance that results in excessive calorie storage as fat (in adipose tissue, liver, and other sites). To fully evaluate the complexities of energetics and regulation of metabolic homeostasis, animal models are frequently employed since energetics measurements can be correlated to intensive evaluations of biology from the whole-organism down to tissue and metabolite levels. Optimally, studies using these models should include measures of a variety of factors that influence net calorie balance and the utilization of carbohydrate, fat, or protein for energy, including, e.g., (a) calorie intake rates and meal frequencies, (b) energy expenditure and behavior- activity levels that contribute to calorie demands, and (c) respiratory gas exchange ratios (CO2 output vs. O2 intake) to calculate whole-body tissue macronutrient fuel choice. In this application, we propose to complement existing UC Davis campus strengths in diabetes and obesity research, exercise physiology and muscle biology, and nutrition and energetics through the purchase of a multi-user device with comprehensive capabilities to simultaneously and automatically record food intake/meal frequencies, activity levels, and metabolic rate and fuel choice. The latter can be determined in a home cage, in the course of physical activity shifts or exercise, or with changes in environmental temperature. This unit has the advantage of being able to accommodate large numbers of mice or rats, thus enabling individual investigators to complete statistically-robust studies in relatively short periods of time, and allowing for broader-based metabolic phenotyping efforts in genetically-modified mouse models in support of the UC Davis Mouse Biology Program mission area. Nine NIH funded faculty with 14 grants active at the time of award are named major and minor users of this equipment.